I noticed (after extracting the default theme stylesheets) that there are supposed to be 3 different colour schemes for the web developer tools in SeaMonkey 2.45, but I can see no way to change the theme when using the browser console. How do you change the theme?

Click the "cog" icon in the top right corner of the developer tools frame/window. If you mouse over the cog, the tooltip says Toolbox Options. One of the sections is labeled Themes, in which you can select "Dark", "Light" or "Firebug".